Dictionary.net

1. In one of its original meanings, this term denoted a tribute paid by English dwellers along the Scottish border to influential chieftains of Scotland, as a condition of securing immunity from raids of marauders and border thieves. 2. It also designated rents payable in cattle, grain, work, and the like. Such rents were called "black-mail," (rcditus nigri,) in distinction from white rents, (blanche firmes,) which were rents paid in silver. 3. The extortion of money by threats or overtures towards criminal prosecution or the destruction of a man's reputation or social standing. In common parlance, the term is equivalent to, and synonymous with, "extortion,"

n. A rate of money, cattle, or produce, paid to secure protection from pillage;—extortion by threats.